As a dopant for conductive polymers, sulfonic acid has been used. That is, low-molecular-weight sulfonic acid and/or high-molecular-weight sulfonic acid have been used (see, for example, Patent Documents 1 and 2). It has been known that high-molecular-weight sulfonic acid maintains a stable interaction with a conductive polymer, in comparison with low-molecular-weight sulfonic acid (see, for example, Patent Document 3). It has been known that, in particular, polystyrene sulfonic acid (hereinafter also abbreviated as “PSS”) exhibits high performance as a dopant (see, for example, Patent Documents 2 and 4). Moreover, polyvinyl sulfonic acid (hereinafter also abbreviated as “PVS”) consisting of a polyethylene main chain that maintains a polymer structure and a sulfonic acid group acting as a dopant exhibits high performance as a dopant for conductive polymers (see, for example, Patent Document 5).